<!doctype html>
<html>
	<head>
		<style>
			body{background:rgba(255,0,0,1)}
			div{width:300px;
				height:300px;
				background:rgba(0,0,255,0.5);
				position:absolute;
				top:0;
				left:0;
			}
		</style>
	</head>
	<body>
		猜猜我在哪里
		<div></div>
	</body>
</html>